500 Internal Server Error and Session Timeout Error

When we entry a big Repair order in CRM web-ui, make a small change and save, we are getting 500 internal server error. No other information in the error. When I look into SM21 below errors I see. Most of the errors are for RFCUSER and some for end users.

As per your problem description this might be an http session handling issue which occurs in IC multisession scenarios. Can you follow the KBA note which addresses the error.

1597836 - HTTP session handling in IC multi session scenarios

It is recommended to set paramter is/HTTP/show_detailed_errors to TRUE. This will send an error message like "The maximum number of sessions has been reached" to the browser instead of a generic HTTP 500 internal server error. For further details you might also check SAP Note 1427190.